It was Kauto Star's sheer wellbeing that caused the problem. Paul Nicholls, his trainer, shows irritation as he relives the moment, just over a fortnight ago, when the steeplechaser's joie de vivre during a practice session at his Somerset stable put his chance of a third Cheltenham Gold Cup success in jeopardy.

"He was just pissing about, larking about, acting a prat and trying to kick everything," Nicholls recalls. It is not a new development. Those who have backed Kauto Star since he first ran in Britain more than seven years ago will remember races that were sewn up until he let his attention drift and jumped into the final fence rather than over it.

"Half his career, it's why I put a sheepskin noseband on him, because he wasn't concentrating," Nicholls adds. "And he was doing that again, not concentrating on what he was doing, made an error, tried to get out of it but couldn't. Upside down, he went. He was physically shaken afterwards, just in shock."

It was a much happier Kauto Star who coasted round Wincanton after racing finished there on Friday, proving his wellbeing to a relieved crowd. He was declared "100% sound" on Saturday and, provided he sails through a final schooling session on Monday, is as likely to make it to the starting line as anything else in the field for Friday's Gold Cup, the highlight of the Cheltenham Festival.

At the time, Kauto Star's tumble was just the latest in a series of disasters for Nicholls, including injuries to a number of high-profile animals and a bout of coughing in his stable that "seemed to sweep round like bloody wildfire". But even such traumas can be put into perspective over a cup of tea in a quiet room, 100 miles from the stable and six days before the Festival's first race.

"It's been interesting," is the trainer's wry assessment, as he sinks into a chair at the West End home of a very rich man, the owner of many racehorses. It is possibly the last bit of peace he can expect before next Saturday.

"I suppose you could say we've been lucky in a number of years when all the big ones have met their targets and not had any problems. This year, it's all just been a bit of a nightmare."

The coughing must have been especially alarming for a trainer who takes such elaborate precautions against infection and who has been able to avoid the viruses that have blighted other yards for entire seasons. But he was able to contain it and the worst was over after 10 days. "I don't see it being too much of a problem," he says, "because they all look so well and they were never sick.

"It might turn out to be the best Festival ever, you just don't know. That's how it is when you have a huge number of horses. Every night, you go on your rounds and you're fraught with what you might find."

Still a month shy of his 50th birthday, Nicholls is one of the four most successful trainers in Festival history, with 30 winners in the past 13 years. His contenders this time include Big Buck's, trying to land the World Hurdle for a record fourth time, Zarkandar, the unbeaten second-favourite for the Champion Hurdle, and Al Ferof, who will attempt to repeat last year's victory over the much-vaunted Sprinter Sacre.

"It's probably not as strong a squad as some years we've had, but quite a lot of them have good chances. I'm looking forward to it."

Thanks to Kauto Star's self-inflicted wounds, that anticipation is shot through with anxiety and not a little frustration because Nicholls clearly feels the horse would have had an excellent chance in the Gold Cup with a clear run. "Before this little hiccup, I would have said he's in as good form as he's ever been.

"His King George win this year was one of his best ever performances. And his Haydock win was brilliant. I think he's probably been in the form of his life this year, at the peak of his powers. This has taken the wind out of our sails a little bit."

There is no denying that Kauto Star has been better this season than last, when Nicholls heard him making a noise on the gallops that suggested a breathing problem. For no evident reason, the noise disappeared this winter and he has attacked his work up the vertiginous Ditcheat hill with renewed relish. "He just looks better, everything about him, he's stronger."

The trainer is now inclined to think that Kauto Star may have taken a full year to recover from his shocking fall in the 2010 Gold Cup, when, for a brief moment, his neck twisted against the turf under the weight of his entire body.

Nicholls describes how, "for the best part of two months", the horse couldn't lower his head to his feed bucket, which had to be placed on a pile of tyres until he could flex his neck once more. "He was in a bad way for a while and it might have taken longer than we think to get over it, mentally and physically. That's why he might be a bit sore now, an old injury just flamed up a little bit.

"To win those races at a high level, everything has to be right and I don't think he was quite right last year [when he was third behind Long Run], or the year when he fell, but he certainly seems OK now. If he could just run his race, it'd be fantastic. I'd be awfully proud if he just turned up at Cheltenham in good order. Just to be there, because they'll cheer him before he runs on Friday, like they did at Haydock. People will just go there to see him."

Nicholls calls Kauto Star "the horse of a lifetime" and struggles to accept that he might not be the best there has ever been, though he has no memory of Arkle, a triple Gold Cup winner in the 1960s regarded by many as an unmatchable giant. A child at the time, he also missed out on the 1969 Gold Cup, won by What A Myth, the last horse of Kauto Star's advanced age, 12, to land the race.

It is the sort of detail that fascinates punters but the trainer could hardly be less impressed. "Statistics are there to be broken," he says and points out that no runner as young as five had won the Champion Chase before he sent out Master Minded to do so in 2008.

Mention of Master Minded provokes further reflection because he and Denman, two of the best to pass through Nicholls's yard, have been forced out of the sport by injuries. Kauto Star's career is not expected to last into 2013 and the trainer can even imagine him being retired on the spot if he wins on Friday.

"It is nearly the end of an era, without a shadow of a doubt," he says, "and now I'm focusing on trying to get the next generation. But we were so incredibly lucky to have had all of those that, if I didn't get any more, I'd have nearly outrun my luck."

It might be imagined that, in these straitened times, there would be limited competition for the right to buy burly steeplechasers, prone to injury and without any breeding potential, but interest is apparently keener than ever. Nicholls's experience suggests that every other millionaire would like to own a Cheltenham winner, their attention perhaps caught by the achievements of his runners over the past decade.

"It's bloody hard to buy Kautos and Denmans and Master Mindeds now. There was a time, five or six years ago, there weren't too many people in the market. Now, if those horses come up, there's nine or 10 people trying to buy them. They're hard to come by."